The past decade has witnessed an explosion in demand for increasingly powerful electronic products with smaller and smaller packages. In response, manufacturers have incorporated high density component assemblies into their products; but limits were soon reached as to improvements in product cost, packaging density, circuit performance, and reliability in using conventional through-hole packaged component assembly technology. Therefore, manufacturers invoked a trend towards the use of surface mount devices in their designs in the mid-1980's.
Surface mountable devices do not require component mounting or lead holes for board affixation, and their packaging size is relatively smaller than that of their pin-through cousins. With this technology, manufacturers are able to reduce overall circuit board assembly size through increased placement densities, finer pitch component terminals, and dual-sided component mounting. Further, the use of smaller mass surface mount technology (SMT) components increases vibration and shock resistance. Also, smaller component sizes and increased placement density mean shorter distances for signals to travel, thereby improving high speed, high frequency circuit performance.
More recently, component manufacturers have introduced a type of surface-mountable component chip package, known as ball grid array or BGA, which discards the use of external leads in integrated circuit chip packaging altogether. Connections between the BGA device and the circuit board are made through contact pads placed on the underside of the chip. Visually, the arrangement of contact pads appears as a two-dimensional grid or matrix of small solder blobs or balls extending from the underside of the chip, hence the name Ball Grid Array. A complementary contact pad array or landing area is located on a surface of the circuit board where the BGA device is to be positioned. Since no leads extend from the BGA package, its effective footprint is appreciably smaller than that of comparable dual or quad flat package SMT designs.
Since typically the BGA footprint is so compact and its lead count and density are so high, it is necessary in manufacturing to place the BGA on the landing area of the circuit board with the solder balls aligned with their corresponding contact pad. Then the entire BGA is heated by blowing hot gas onto the top of the BGA. The heat is transferred in a conductive heating process through the BGA to the solder balls until the solder balls reflow. When reflow occurs, the BGA visibly lowers closer to the circuit board due to the melted solder balls.
Because of the need to transfer the heat through the BGA in order to reflow the solder balls, there is a loss of heating efficiency in this technique. The BGA acts as a heat sink so that the solder balls are only indirectly heated by the hot gas. It would be more desirable to heat the solder balls directly by the hot gas.
Another method of placing the BGA component on the circuit board involves the use of a pivot arm which holds the component above and away from the board. The pivot arm holds the component at an angle to the circuit board up in the air high enough for a nozzle to be positioned to blow hot gas at the bottom of the component. This nozzle is separate from the pivot arm. Thus, the nozzle is not the part that holds the component. The nozzle points at the solder balls and blows a stream of hot gas directly onto them until the solder balls reflow. Thus, the stream of hot gas is not guided by an airflow passageway between the nozzle and the component. Then the nozzle quickly moves out of the way, and the pivot arm quickly pivots down and places the component on the board. The arm then releases the component and pivots back up out of the way. This method requires additional machinery to accomplish the same task as the conductive heating process, and the nozzle retraction, pivot down and place operations must take place quickly to allow the solder balls to remain molten, further complicating the machinery. Thus, this method increases the complexity of the process, but has the advantage of only heating the solder balls, leaving the BGA and the circuit board much cooler.
Another method of placing a BGA component on a circuit board is to set the component directly onto the landing area of the circuit board and to use a special nozzle for blowing hot air under the component. This method, however, provides non-uniform heating of the solder balls and does not permit easy airflow due to the very small space between the component and the board.
Therefore, it is necessary to provide a method and apparatus for assembling or reworking printed circuit boards that directly heats the solder balls of a BGA component with a minimum of complexity in the apparatus.